UNBLEACHABLE RHODOPSIN WITH AN 11-CIS-LOCKED 8-MEMBERED RING RETINAL - THE VISUAL TRANSDUCTION PROCESS

Visual transduction occurs through photorhodopsin, the primary photoproduct of rhodopsin, which relaxes to bathorhodopsin and a series of other intermediates until it reaches the metarhodopsin II stage, upon which the enzymatic cascade leading to vision is activated. Despite advances in areas related to visual transduction, the triggering process itself, a key problem in the chemistry of rhodopsin, has remained unsolved. In order to clarify the extent of involvement of the chromophoric excited state versus the 11-cis to trans isomerization, and as an extension of past studies with 11-cis-locked seven-membered ring rhodopsin (Rh7), 11-cis eight- and nine-membered ring retinal analogs, ret8 and ret9, respectively, have been synthesized. The bulkiness of the tetramethylene bridge in ret8 led to numerous unexpected obstacles in attempts to reconstitute a ret8-containing rhodopsin (Rh8) embedded in lipid bilayer membranes. These obstacles were solved by using methylated rhodopsin which gave MeRh8 containing 11-cis-ret8 as its chromophore. MeRh8 exhibited UV-vis and CD spectra very similar to those of native rhodopsin (Rh); furthermore, the quantum efficiency of photorhodopsin formation was comparable to that of Rh. Flash photolytic studies of Rh8 and other ring analogs [Mizukami, T., Kandori, H., Shichida, Y., Chen, A.-H., Derguini, F., Caldwell, C. G., Bigge, C. F., Nakanishi, K., & Yoshizawa, T. (1993) Proc. Natl. Acad. Sci. U.S.A. 90,4072-4076] coupled with the present enzymatic studies with MeRh8 and a series of dihydro-rhodopsins have led to the conclusion that (i) charge translocation in the excited state does occur; however, (ii) full cis-trans isomerization around 11-ene involving the entire polyene moiety is required for efficient transduction to occur. Repeated attempts to incorporate ret9 into opsin have as yet not been successful.